Cover C++20 in CI (fixing char8_t-related problem there).

.github/workflows/ci.yml

@@ -23,6 +23,7 @@ jobs:
       matrix:
         rust: [nightly, beta, stable, 1.82.0, 1.77.0, 1.74.0, 1.73.0]
         os: [ubuntu]
+        flags: ['']
         include:
           - name: Cargo on macOS
             rust: nightly
@@ -31,6 +32,18 @@ jobs:
             rust: nightly-x86_64-pc-windows-msvc
             os: windows
             flags: /EHsc
+          - name: C++14
+            rust: nightly
+            os: ubuntu
+            flags: -std=c++14
+          - name: C++17
+            rust: nightly
+            os: ubuntu
+            flags: -std=c++17
+          - name: C++20
+            rust: nightly
+            os: ubuntu
+            flags: -std=c++20
     env:
       CXXFLAGS: ${{matrix.flags}}
       RUSTFLAGS: --cfg deny_warnings -Dwarnings

include/cxx.h

@@ -45,6 +45,13 @@ class String final {
   String(const char *, std::size_t);
   String(const char16_t *);
   String(const char16_t *, std::size_t);
+#if __cplusplus >= 202002L
+  // `reinterpret_cast` of `const char*` into `const char8_t*` is dangerous,
+  // because `const char*` may point to non-UTF8.  OTOH, conversion in the
+  // other direction seems safe.
+  String(const char8_t *s) : String(reinterpret_cast<const char*>(s)) {}
+  String(const char8_t *s, std::size_t len) : String(reinterpret_cast<const char*>(s), len) {}
+#endif
 
   // Replace invalid Unicode data with the replacement character (U+FFFD).
   static String lossy(const std::string &) noexcept;

tests/ffi/tests.cc

@@ -878,7 +878,8 @@ extern "C" const char *cxx_run_test() noexcept {
   ASSERT(cstr == "foo");
   ASSERT(other_cstr == "test");
 
-  const char *utf8_literal = u8"Test string";
+  // `u8"foo"` is `const char*` before, and `const char8_t*` after C++ 20, so using `auto`.
+  const auto *utf8_literal = u8"Test string";
   const char16_t *utf16_literal = u"Test string";
   rust::String utf8_rstring = utf8_literal;
   rust::String utf16_rstring = utf16_literal;